1. Field of the Invention
The invention relates generally to modems and more particularly to modems employing transmission media subject to echoes.
2. Description of the Prior Art
Integrated services digital network (ISDN) service is a service offered by telephone companies in which subscribers are offered data as well as voice transmission services. A major barrier to the expansion of ISDN service has been the unavailability of ISDN modems which combine low cost with high performance and low power consumption.
One reason for the unavailability of such modems has been the demanding environment in which such modems operate. The modems employed in ISDN must communicate across a telephone subscriber loop. The loop may be up to 18 kilofeet long, may be made of a number of different cable types, and may include a number of bridge taps. In this environment, a signal being received in a modem may be 30-40 decibels lower than the echo of the signal produced by the modem itself as it transmits. The faintness of the received signal in comparison with the echo requires that any ISDN modem used on a subscriber loop have very high quality echo cancellation. A particular problem with providing such echo cancellation for modems used in subscriber loops is that the properties of the subscriber loop as a transmission medium vary over time. As the properties of the subscriber loop vary, the timing of the signals received by the modem changes. The modem 's internal timing must be synchronized with the timing of the signals, and consequently, the modem must occassionally adjust its internal timing.
The best technique available for synchronizing the modem's internal timing with the timing of the signals is the digital phase locked loop. Digital phase locked loops are easily implementable in integrated circuits, and the implementations are easily manufactured and tested. Digital phase locked loops are thus the technique of choice in low-cost modems. However, digital phase locked loops make echo cancellation more difficult. When a digital phase locked loop is employed, timing changes are made in discrete steps. Such changes add timing jitter to the echo cancellation, and the echo cancellation is degraded unless the timing jitter is taken into account. See D. G. Messerschmitt, "Asynchronous and timing jitter insensitive data echo cancellation," IEEE Trans. Commun., vol. COM-34, December 1987, pp. 1209-1217. One mason for the unavailability of low-cost ISDN modems suitable for use in subscriber loops has been the lack of low-cost techniques for compensating for timing jitter.
It is an object of the apparatus and methods disclosed herein to provide a high-performance modem with improved manufacturability and testability. In the course of attaining this objective, other objectives have been attained, including effective and low-cost echo cancellation in the presence of timing jitter, jitter compensation employing continuous adaptation of coefficients, timing circuits which permit the use of low-cost, wide tolerance crystals, and improved decimator circuits.